An organic light-emitting device, also referred to as an organic electroluminescent device, can be constructed by sandwiching two or more organic layers between first and second electrodes.
In a passive matrix organic light-emitting device (OLED) of conventional construction, a plurality of laterally spaced light-transmissive anodes, for example indium-tin-oxide (ITO) anodes, are formed as first electrodes on a light-transmissive substrate such as, for example, a glass substrate. Two or more organic layers are then formed successively by vapor deposition of respective organic materials from respective sources, within a chamber held at reduced pressure, typically less than 10−3 torr (1.33×10−1 pascal). In addition to doped or undoped organic light-emitting material, typical organic layers used in making OLED displays are doped or undoped organic hole-injecting material, doped or undoped organic hole-transporting material, and doped or undoped organic electron-transporting material, where doping refers to adding a minor constituent to enhance the electrical performance, optical performance, stability, or life time of a given material or device constructed thereof A plurality of laterally spaced cathodes is deposited as second electrodes over an uppermost one of the organic layers. The cathodes are oriented at an angle, typically at a right angle, with respect to the anodes.
Applying an electrical potential (also referred to as a drive voltage) operates such conventional passive matrix organic light-emitting devices between appropriate columns (anodes) and, sequentially, each row (cathode). When a cathode is biased negatively with respect to an anode, light is emitted from a pixel defined by an overlap area of the cathode and the anode, and emitted light reaches an observer through the anode and the substrate.
In an active matrix organic light-emitting device (OLED), an array of anodes are provided as first electrodes by thin-film transistors (TFTs) which are connected to a respective light-transmissive portion. Two or more organic layers are formed successively by vapor deposition in a manner substantially equivalent to the construction of the aforementioned passive matrix device. A common cathode is deposited as a second electrode over an uppermost one of the organic layers. The construction and function of an active matrix organic light-emitting device is described in commonly-assigned U.S. Pat. No. 5,550,066, the disclosure of which is herein incorporated by reference.
Organic materials, thicknesses of vapor-deposited organic layers, and layer configurations, useful in constructing an organic light-emitting device, are described, for example, in commonly-assigned U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432, and 4,769,292, the disclosures of which are herein incorporated by reference.
Other kinds of imaging devices, such as imaging phosphors for computed radiography and x-ray photoconductive devices for digital radiography, depend on the ability to coat the active materials uniformly over large areas. While the following discussion pertains to OLED displays, it should be readily apparent that the same invention can be applied to the deposition of alkalihalide phosphors, amorphous semiconductors, and other luminescent or photoactive layers, as well as a variety of other materials used in devices based on such luminescence or photoactive layers.
For sufficiently small substrates, a point source approach can be implemented wherein the material to be deposited emanates from a localized heated crucible and the substrate is placed sufficiently far from the localized region of vaporization that the coating is sufficiently far from the localized region of vaporization that the coating is sufficiently uniform along the substrate. As substrate size increases or working distance increases, rotary or planetary motion of the substrate relative to the localized source is often required to produce the desired uniformity.
By elongating the vaporization source and providing for translation of source and substrate relative to one another, the desired uniformity can be attained at considerably smaller working distances and thus considerably higher rates and better materials utilization, if desired. Scaling of such a process to large areas (i.e. substrates greater than 15 cm in at least one dimension) is considerably easier than for point sources.
An elongated source for thermal physical vapor deposition of organic layers onto a structure for making an organic light-emitting device has been disclosed by Spahn in commonly assigned U.S. Pat. No. 6,237,529. The source disclosed by Spahn includes a housing, which defines an enclosure for receiving solid organic material, which can be vaporized. The housing is further defined by a top plate which defines a vapor efflux slit-aperture for permitting organic vapors to pass through the slit onto a surface of a structure spaced apart from the elongated source. The housing defining the enclosure is connected to the top plate. The source disclosed by Spahn further includes a conductive baffle member attached to the top plate. This baffle member provides line-of-sight covering of the slit in the top plate so that organic vapors can pass around the baffle member and through the slit onto the substrate or structure while particles of organic materials are prevented from passing through the slit by the baffle member when an electrical potential is applied to the housing to cause heat to be applied to the solid organic material in the enclosure causing the solid organic material to vaporize.
In using the thermal physical vapor deposition source disclosed by Spahn to form an organic layer of a selected organic material on a substrate or structure, it has been found that the vapor efflux slit-aperture causes non-uniform vapor flux of organic material to emanate along a length dimension of the slit. There is a problem when the width dimension of the slit is reduced, for example, to a width dimension less than 0.5 mm. Such spatially non-uniform orientation of opposing slit edges can be thought of as a deviation of planarity of opposing edges which, in turn, can promote a greater fraction of organic vapors to exit the vapor deposition source through a central portion of the slit, with a correspondingly lower fraction of organic vapors exiting the source through remaining portions of the slit along its length dimension. Such non-uniform vapor flux, directed at a substrate or structure, will cause the formation of an organic layer thereon which will have a non-uniform layer thickness in correspondence with the non-uniform vapor flux.
In addition, any nonuniformities in heat generation from the heater or heat absorption by the material to be deposited or distribution of the material within the source can give rise to nonuniformity in deposition along the length of the source. Yet another source of nonuniformity is unintended leaks in the source enclosure other than the apertures used to deliver the organic vapor. If such leak exists at the ends of the source, the flow of vapor from center to end of the source can cause pressure gradients within the source, thereby causing nonuniformity in the resultant deposition.
Forrest et al (U.S. Pat. No. 6,337,102B1) disclosed a method of vaporizing organic materials and organic precursors and delivering them to a reactor vessel wherein the substrate is situated and delivery of the vapors generated from solids or liquids is accomplished by use of carrier gases. In one embodiment of their invention, Forrest et al located the substrates within a suitably large reactor vessel, and the vapors carried thereto mix and react or condense on the substrate. Another embodiment of their invention is directed towards applications involving coating of large area substrates and putting several such deposition processes in serial fashion with one another. For this embodiment, Forrest et al disclosed the use of a gas curtain fed by a gas manifold (defined as “hollow tubes having a line of holes”) in order to form a continuous line of depositing material perpendicular to the direction of substrate travel.
The approach to vapor delivery as disclosed by Forrest et al can be characterized as “remote vaporization” wherein a material is converted to vapor in an thermal physical deposition source external to the deposition zone and more likely external to the deposition chamber. Organic vapors alone or in combination with carrier gases are conveyed into the deposition chamber and ultimately to the substrate surface. Great care must be taken using this approach to avoid unwanted condensation in the delivery lines by use of appropriate heating methods. This problem becomes even more critical when contemplating the use of inorganic materials that vaporize to the desired extent at substantially higher temperatures. Furthermore, the delivery of the organic vapor for coating large areas uniformly requires the use of gas manifolds.
Each one, or a combination, of the aforementioned aspects of organic powders, flakes, or granules can lead to nonuniform heating of such organic materials in physical vapor deposition sources with attendant spatially non-uniform sublimation or vaporization of organic material and can, therefore, result in potentially non-uniform vapor-deposited organic layers formed on a structure.